What loss? The performance and functionality of the heatsink is not diminished. You bought the heatsink with the intention of using it, correct? Does the discolouration on the bottom of the heatsink affect your ability to use it or its ability to cool your CPU adequately? No.

You're just trying to milk him for some cash. If the heatsink was broken or something was preventing it from doing its job then you might have a complaint. However, this is an incredibly minor issue. You need to accept that not everything in life is going to be perfect and sometimes you just have to leave well enough alone. I'm sure that by now Crossfire has already decided that he isn't going to give you a refund, so none of your complaining is going to amount to anything.